Hazardous fluids are routinely conveyed within enclosed pipes or conduits. Recognizing that such pipes and conduits can leak due to manufacturing defects, excessive pressure, corrosion, and joint defects, for example, which may be due to thermal stresses, double-containment piping systems have been developed in which an inner or primary pipe is located within an outer or containment pipe forming an annulus between the inner and outer pipes. The annulus is typically dry, except in the event of a leak of fluid carried by the inner pipe.
Double-containment piping systems are now widely used, which is due in part to the enactment in the United States of improvements to the Resource Conservation and Recovery Act in 1988 and to the Clean Water Act in 1990. For example, double-containment piping systems are now used for such applications as gasoline-station piping, fuel-oil piping for underground storage tanks that supply fuel oil for heating systems in buildings, chemical piping in a variety of applications, chemical sewers, process sewers and landfill systems.
In many known double-containment piping systems, the inner pipe is typically supported by resting directly on the lower, inside wall of the outer pipe. Alternatively, support apparatus are provided in which the inner pipe is supported in spaced relation to the inside wall of the outer pipe. Exemplary supports are illustrated in the following U.S. patents: U.S. Pat. Nos. 5,141,184; 5,018,260; 4,751,945; 3,863,679; and 3,417,785.
Although such prior supports may be used to support the inner pipe within the outer pipe, and provide ready access to the inner pipe at spaced locations along the outer pipe, such supports are typically joined to the inner pipe and/or the outer pipe by welding or bonding means to secure the support in place. The welding or bonding process can require significant construction and assembly time, which results in added project costs, and further prolongs the installation time of the double-containment piping system. The welding or bonding process also typically requires that the support be made of the same material as either the inner or the outer pipe so that it can be adequately welded or bonded to the respective pipe. Accordingly, any flexibility in selecting the material for the support is frequently limited by the materials of the inner and outer pipes.
Prior supports also frequently directly support an inadequate portion of the inner pipe in comparison to that which is typically required for single-wall above-ground pipes. For example, there are one-piece collar-type supports which by design fit less than perfectly around the external circumference of the inner pipe, in order to slip the support into position over the inner pipe. This type of collar support is then secured in place typically by welding or otherwise bonding the support to the inner pipe. Thus, although such a support may surround the inner pipe, it typically does not maintain sufficient support of the entire circumference of the inner pipe because its inner diameter is inherently greater than the outer diameter of the inner pipe. This is a particular disadvantage with fiberglass pipes, which typically require uniform support along a substantial portion of the circumference of the inner pipe.
In some double-containment piping applications, it would be desirable to have multiple inner or primary pipes mounted within a single outer or containment pipe. This may be advantageous, for example, where there are several primary pipes that run in the same direction or otherwise follow the same path or route. If the several primary pipes were mounted within a common containment pipe, then a single means of leak detection could be employed, thus making the overall system more economical. This type of double-containment system may also be desirable where there are space limitations preventing each primary pipe from being individually housed within a respective containment pipe. A typical example for this type of system would be one for supplying fuel oil from a storage tank to several burners in a building basement. In this type of system, it is common to have several supply lines running from the storage tank to the burners, and several return lines, or a single return line running from the burners back to the storage tank. There may also be one or more fuel tanks in series, wherein each tank may have one or more supply and return lines, and all such lines follow essentially the same route between the burners and the tanks. Similar arrangements may be used in gasoline stations for pumping the gasoline between several fuel pumps and one or more underground fuel storage tanks. In such applications, it would be desirable to house a plurality of primary pipes within a single containment pipe, and to have a centering support suitably designed to support each of the individual primary pipes within the single containment pipe. It would also be desirable for such a centering support to maintain the primary pipes in a consistent geometric arrangement throughout the double-containment piping system.
Double-containment piping systems are also employed in natural gas pipelines, particularly where such pipelines pass beneath roadways. In these applications, the primary pipe is housed within an outer casing or containment pipe, which provides structural protection for the primary pipe by isolating the primary pipe from potentially harmful dynamic loads due to vehicular traffic on the roadway. Because the natural gas pipelines and their containment pipes are made of steel, it is important to isolate the primary pipe from the containment pipe with a nonmetallic centralizing device. If the annulus between the primary and containment pipes were to fill with water, the nonmetallic centralizing device may prevent the water from causing galvanic corrosion. In this type of application, the centering support would also function as a "casing insulator".
It is an object of the present invention to overcome the drawbacks and disadvantages of prior art centering supports for double-containment piping systems.